1. Field of Invention
The present invention relates to cellular communication systems; and more particularly to a method and system for effecting handoff of an on-going communication from a serving cell to a neighboring cell using scanning receivers at cell base stations.
2. Description of Related Art
A cellular communication system includes several base transceiver stations, each of which transmits and receives channels of RF information throughout a predetermined coverage area referred to as a cell. The outer boundary of a cell is determined not only by the effective radiated power of the base transceiver, which determines the maximum range at which a mobile unit is able to receive; but also, by the effective radiated power of the mobile telephones, which determines the maximum range at which the base station is able to receive. The base transceiver stations (BTS) determine their corresponding cell's handoff boundaries by setting an RF signal threshold value which must be met or exceeded by a mobile unit entering the cell in order to transfer an on-going call to that cell.
In many cellular installations, each BTS has several scanning receivers that periodically, or on command, measure the RF signal strength of selected traffic channels in order to determine the range of active mobile telephones relative to the transceiver of the cell in which the mobile telephone is located, referred to herein as a serving cell, and the proximity of the telephone to neighboring cells, which may or may not overlap the serving cell.
Based upon the received RF signal strength of the mobile unit by the transceiver of the serving cell, the BTS controls the effective radiated output power (ERP) of the mobile unit so that the transmission by the mobile unit does not saturate the base station transceiver when it is close to the center of the cell, but is of sufficient power to enable the BTS to receive the mobile transmission when it is far from the transceiver. When the RF received signal strength of a mobile unit received by the scanning receiver of a neighboring cell exceeds a fixed nominal RF signal threshold value that is configured into the system; then the system switches the on-going communication to a traffic channel of the neighboring cell, thus, effecting a handoff.
Because of variations in reception and transmission by the mobile units and the BTS at various locations in the cell, which may be caused by geographic phenomena, such as the terrain, or man made obstructions, for example, the scanning receivers are unable to determine the location of a mobile telephone; resulting in either improper or lost handoffs. Also, the use of low power mobile telephones, as well as the differences in size of contiguous cells causes problems in effecting handoffs.
For example, if a mobile unit is within a large cell where a high power or low attenuation level of the mobile is permitted and moves toward a smaller cell where the power output is limited to a relatively high attenuation level, the mobile unit could be handed off to the smaller cell without actually being within the smaller cell. If the mobile unit is within a small cell, where the mobile units's minimum permissible attenuation level is high (low power output) and is moving toward a large cell where the mobile unit's minimum attenuation level is low (high power output), a handoff of the call could be lost. In installations that include microcells within the serving cells and a handoff to a microcell is required, the determination of the proper location may be inaccurate.
In light of the foregoing, there is a need for a method and system of handoff in a cellular communication system which overcomes the drawbacks and shortcomings hereinbefore mentioned.